Due to the increasing integration scale (i.e., microminiaturization) of integrated circuits and other electronic devices (e.g., liquid crystal displays), the performance requirements for projection exposure apparatuses have become more demanding. The preferred avenues for meeting these demands is to increase the numerical aperture (NA) of the projection lens system (hereinafter, "projection lens") and/or decrease the wavelength of light used in the projection exposure apparatus.
Increasing the NA of the projection lens is a challenging problem in lens design because of the difficulty in correcting aberrations, particularly when the size of the exposure field needs to be relatively large. One way to achieve the proper degree of aberration correction is through the use of aspheric lens elements. Also, aspheric lens elements reduce the number of lens elements in the projection lens, which increases transmission and makes the lens lighter. The projection lenses disclosed in Japanese Patent Applications Kokai No. Hei 1-315709, 5-34593, and 7-128592 use aspheric surfaces, but the NA and the size of the exposure field are not sufficiently large.